Demodulating circuit arrangement



A. KAROLUS DEMODULATING CIRCUIT ARRANGEMENT Nov. 14, 1933.

Filed NOV. 30, 1928 a L' ,4 A l y Patented Nov. 14, 1933 UNITED STATES DEMODULA'IING 1,935,153 4 CIRCUIT ARRANGEMENT August Karolus, Leipzig, (rermany,` assignor to 4Radio Corporation of America, a corporation of Delaware *Application November` 30, 1928, Serial N0.

322,754, and in 'Germany December 12, 1927 1 Claim;

This invention relates to the demodulation of i modulated alternating current or, in other words,

its purpose is to separate the modulating frequencies from the modulated or carrier frequency 5 or wave. .The application of the invention in connection with picture telegraphy work brings out the .essential features of thel invention with particular clearness for which reason its use f or picture telegraphy shall herey be especially considered.

For the purpose of insuring undistorted amplification of the frequencies produced in exploring a picture and occupying a wide frequency band, the so calledL carrier wave method has heretofore beenused in picture transmission by telegraphy, in other words, the `picture signals are amplified and transmitted. in the form of modulations of a separately generated carrier wave. For in.- stance, in picture transmitter equipments operating with picture exploration by optical or luminous means, this is effected by that the light brought to act upon the photoelectric cell is periodically interrupted, whereby the carrier frequency is obtained. The modulation of this carrier. frequency obtained by the interruption of light by means vof the frequencies resulting from the distribution of brighter and darker spots in the picture in this case is effected at the-grid of the rst amplier tube. Apart from the method hereinbefore outlined, a number of others based upon purely electrical principles for the production and modulation of `the carrier wave are known in the prior art, for instance, the so called mixed tube schemes or the provision of inter#- rupters for the photoelectric cell current.

The invention hask been illustrated by the accompanying drawing in one of its preferred modifications. By the drawing:

Fig. 1 illustrates the rectification of picture currents according to the prior art;

Fig. 2 illustrates the current curves according to the present invention; K

Fig. 3 shows one modification which the system may assume, utilizing a bridge arrangement forv selecting currents having a phase displacement of 90 with respect to each other; and,

Fig. 4 illustrates a further modification by which n phases may be obtained through the use of an arrangement of the type shown by Fig. 3.

All of these amplification and transmission methods are predicated for their operation and use upon demodulation at the receiving end. A similar process may be needed also for the transition from one transmission system to another system, say, from a cable line to a radio frequency trans- (Cl. 25o-,21)

mitter. In thisr instance, demodulation is resorted to for the reason that intermediate frequency or double modulation of the radio frequency wave, which inV turn becomes the carrier of the picture signals, with the lower carrier frequency usual for amplification and transmission over a cable should be avoided, inasmuch as such doublel modulation is associated with an undesirable enlargement of the frequency band required for the transmission.y

According to former practice, demodulationas herenbefore set forth was mostly effected by Ya rectificationprocess. This, however, invariably results only in a pulsating direct current whose enveloping curve represents the shape or the variation of the signals or, in different terms, represents the distribution of brightness in `the picture being transmitted. i

An illustration of what precedes is illustrated in the ydrawing (Fig. la). The portions of the carrier wave indicated by the dotted line have been cut 01T by the rectification, the dash line indicates the enveloping curve which corresponds to the brightness Variations of the explored picture. Hence, it will be seen that a current curve of this form invariably'contains stillY the carrier frequency, or, under certainv conditions, (that is, for double rectification orgrectification of both halves of the wave) twice the frequency. Double modulation, by which is meant rst, modulation at theV picture lelement rate,` and secondLatthe carrier rate, is thus not attained. To smooth the current curve as described, recourse has heretofore been had to the provision of condensers or of a chain system acting as a lter for the said purpose.

Fig. 1b shows the outcome in the case shown in Fig. la after the current has passed through a filter chain of ideal action. However, one practical drawback in connection with the use of such smoothing means resides in the time constant, and as a result the shape of the signal as corresponding to the picture is distorted, and this, in turn, for a maximum admissible signal distortion manifests itself in an undesirable reduction of the feasible transmission speed.

Now, thev present invention solves the problem of demodulation with the incidental elimination of the carrier wave in the sense as hereinbefore outlined ina fundamentally different manner. According to the method here disclosed the pulsations are suppressed or at least markedly diminished by deriving from the modulated frequency in known manner two or more phase disthese phase displaced alternating currents are separately demodulated or rectified; and, finally, the phase displaced pulsating direct current potentials are combined in some suitable manner. Owing to the ensuing overlapping of the current impulses, a curve characterized by a lower ripple percentage is obtained, the remaining per-` centage depending upon the number of phases that are produced, in other words, it will be so much smaller, the greater the number of phases..

In the simplest case one operates in such a way that two currents roughly in quadrature relation are derived from the modulated carrier frequency, for instance, by that the modulated carrier wave is shorted by an impedance containing a resistance and a capacity in series. In this case, as is well known, the two potentials across the resistance and the capacity are shifted or `dis placed by an angle of 90 degrees. Each of these potentials, if necessary, after amplification, is rectified separately, and the resultant pulsating direct current potentials, as above pointed out, are combined in a joint consumer (for example, at the grid resistance of a tube or directly at the consumer or load resistance).

Fig. 2 may serve to further explain this method: a and b arekthe modulated alternating current potentials presenting a phase displacement of 90 degrees and takenoif across the terminals of the resistance and of the capacity, respectively, c and d are the pulsating direct current potentials of double frequency obtained by rectification of both halves, e indicates the resultant after combination of c and d, and the shape of which shows the resemblance with the enveloping of a or b. Unlike c or d, the strength of the current no longer drops to zero value, indeed, a curve of a lower ripple percentage is obtained. rihe fact that in these overtones the carrier frequency is still clearly distinguishable is merely due to the nonsinuous shape chosen for the rectified waves c and d in the drawing. In practice, on the -contrary, especially the higher harmonics are far more flattened than has been shown in the drawing. As to the rest, they are readily eliminable as shall later be shown. f

instead of the combination comprising a resistance and a capacity vas here shown, also any other convenient combination of reactances may be employed for obtaining potentials of any desired number of phases and phase angle. These reactances may preferably be connected bridgefashion as shown in Fig. 3. The modulated alternating current potential E0 is supplied by way of conductor Ll L2 to a bridge comprising the resistances Rl R2 and the capacities C1 C2. The

suppressed by filters.

Vthen utilized for the control of phase displaced modulated alternating current which are rectified each in a distinct system by a both-half rectification scheme.

VThe derivation of phase displaced potentials could also be effected by the use of rotary fields by 'that the alternating current potential to be demodulated is first split in known manner so that a rotating field is obtained, the latter thereupon producing by induction in a system comprising coils conveniently disposed an alternatingcurrent of n phases, as shown by Fig. 4. As to the rest, so far as the object of the invention is concerned, fundamentally all such methods known from power work are useful as insure a `change from a given number of phases to n many phases.

Suchtremnants or traces of frequencies as remain from the circuit arrangement hereinbefore described and as are to be eliminated by filtering, provided they prove troublesome, can be However, contradistinct from the arrangement hereinbefore referred to these filters may be designed that there results a practically far lower time'constant. Hence, they occasion no appreciable distortion of the signals to be transmitted and they do not restrict the transmission speed.

Although in whatprecedes, the description of i5 the invention has been applied particularly to the case of picture transmission, it will be understood that the present disclosure is not confined to this application. Indeed, fundamentally speaking, it is useful for all other purposes in the ijs, communication arts whenever carrier waves are used for the transmission.'

Having now described my invention, I claim: The method of demodulating carrier frequencies which comprises, deriving a plurality of 325 phase displaced potentials from the modulated carrier frequency, rectifying the phase displaced carrier potentials, and recombining the separate phase displaced pulsating direct current potentials.

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f isc AUGUST KARoLUs. 

